1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device having reverse conducting faculty comprising a switching element including a semiconductor substrate of a first conductivity type having first and second major surfaces, a first main electrode region of the first conductivity type formed on the first major surface of the semiconductor substrate, a second major electrode region of a second conductivity type formed on the second surface of the semiconductor substrate, and a control electrode region of the second conductivity type for controlling a current passing between the first and second main electrode regions.
2. Description of the Related Art
As a voltage supply source for a pulse laser and pulse discharge device, there has been used a voltage supply source generating a pulse having a high voltage and a large current. FIG. I shows an example of a known pulse generating circuit used as a voltage supply source for use in a pulse laser. In this pulse generating circuit, between output terminals 14a and 14b of a charging circuit 14 including a DC power sur)ply source 11, a switch 12 and a current limiting resistor 13, is connected a static induction thyristor 15 (herein after abbreviated as SIThy). In parallel with the SIThy 15, there are connected resonance coil 16 and capacitor 17. Furthermore, in parallel with the capacitor 17 are connected a capacitor 18 and a coil 19 having a large inductance, and a discharge gap 20 is connected across the coil 19 as a load.
Under a non-conductive condition of the SIThy 15, at first the switch 12 is closed to charge the capacitor 17 through the resistor 13 and coil 16. During this charging process, an impedance of the coil 19 at a lower frequency is low, and thus the capacitor 18 is also charged through the coil 19. Now an output voltage of the DC power supply source 11 is denoted by E. After charging the capacitors 17 and 18 up to E, the SIThy 15 is turned-on by means of a gate driving circuit 21. Then, charge stored in the capacitor 17 is discharged through the SIThy 15 in accordance with a resonance characteristic determined by the coil 16 and capacitor 17, and the capacitor 17 is charged in a reverse polarity to a polarity in which the capacitor 17 is charged up to substantially xe2x88x92E. Charge stored in the capacitor 18 is also discharged through the SIThy 15 and coil 19. Since an impedance of the coil 19 is very high for a high frequency, the discharge is conducted very slowly. Therefore, a voltage of about xe2x88x922E will be applied across the discharge gap 20. When a discharge occurs, charge stored in the capacitors 17 and 18 disappears by discharge at the discharge gap 20. And the switch 12 is closed to initiate the charging operation again.
In the above mentioned pulse generating circuit, if the discharging operation is carried out correctly between the discharge gap 20 when a voltage of xe2x88x922E is applied across the discharge gap, charge stored in a resonance circuit consisting of the coil 16 and capacitor 17 disappears. Therefor, as shown by a solid line in FIG. 2, no current flows through the SIThy 15 in the reverse direction. However, if discharge does not occur correctly due to any reason, a ringing current occurs in the resonance circuit and a large current flows through the SIThy 15 in the reverse direction as illustrated by a broken line in FIG. 2.
FIG. 3 is a graph showing a voltage across the anode-cathode path of the SIThy 15. When discharge does not occurs correctly, a reverse voltage is applied to the SIThy 15. In this case, a reverse current flows from the cathode to the gate of the SIThy 15, and this results in application of an excessive high reverse voltage like as a reverse recovery phenomenon of the diode.
In order to protect the static induction thyristor from the breakdown when the large reverse current flows through the anode-cathode path of the thyristor, it has been proposed to flow the reverse current through a diode connected in anti-parallel with the static induction thyristor. The static induction thyristor having such a diode is generally called a reverse conducting static induction thyristor. In the reverse conducting static induction thyristor, in order to make a wiring inductance as small as possible, it has been proposed to form the diode by a common semiconductor substrate together with the static induction thyristor in a preliminary thesis issued for 1999 Conference of the Electric Engineering Society by Shimizu et al., xe2x80x9c4000V Class Reverse Conducting SI Thyristor(1)xe2x80x9d.
FIG. 4 is an equivalent circuit of the above mentioned reverse conducting static induction thyristor. A diode 32 is connected in anti-parallel with a static induction thyristor (SIThy) 31 such that an anode of the diode is connected to a cathode of the SIThy and a cathode of the diode is connected to an anode of the SIThy. The anode of the diode 32 is further connected to a gate of the SIThy 31 by means of a resistor 33, and the gate of the SIThy is connected to a gate driving circuit (GC) 34 which controls the turn-on/turn-off of the SIThy. When a main power supply source 35 is connected across the anode-cathode path of the SIThy 31 as shown by a solid line in FIG. 4, a current IT flows through the SIThy, and when a voltage supply source 36 is connected in a reverse polarity as depicted by a broken line in FIG. 4, a current IR flows through the diode 32 to protect the SIThy 31 from being breakdown.
FIG. 5 is a cross sectional view showing the structure of the above mentioned known reverse conducting static induction thyristor. In one major surface of an nxe2x88x92 silicon substrate 41 there is formed a p+ gate regions 42, and p+ buried gate regions 43 are formed within a channel region. A gate electrode 45 is provided on the gate region 42 via a conductive layer 45a. The buried gate regions 43 are formed as a comb shape to be surrounded by the gate region 42. Above the channel region, there are formed n+ cathode regions 46 which are electrically connected to a cathode electrode 47 via a conductive layer 47a. On the other major surface of the silicon substrate 41, an anode electrode 52 is provided via a conductive layer 52a. In this manner, a thyristor section 44 is constructed by the gate region 42, buried gate regions 43, channel region, cathode regions 46. Furthermore, a diode section 49 is formed to surround the thyristor section 44 via a separation band 48. The diode second includes a p+ anode region 50 and a cathode region 41a formed by a part of the nxe2x88x92 silicon substrate 41. The anode region 50 is electrically connected to the cathode electrode 47 of the static induction thyristor via a conductive layer 47a and the cathode region 41a is connected to an anode electrode 52 of the static induction thyristor by means of n+ contact region 51 and conductive layer 52a. 
In the above explained reverse conducting static induction thyristor, when a reverse voltage is applied across the anode-cathode main current path, the diode section 49 is made conductive to prevent the thyristor section 44 from the breakdown. However, when the known reverse conducting thyristor is used in the above mentioned pulse generating circuit shown in FIG. 1, the static induction thyristor is often broken by the ringing current generated in the resonance circuit by failure of discharge. In order to investigate a mechanism of such a phenomenon, the inventors have conducted a detailed analysis about the influence of the application of the reverse voltage across the anode-cathode path of the reverse conducting static induction thyristor.
FIGS. 6, 7 and 8 are graphs showing the operation of the static induction thyristor used in the pulse generating circuit upon occurrence of discharge failure. FIG. 6 represent a variation of a current Iak flowing through the anode-cathode path, FIG. 7 shows a variation of a gate current Ig and FIG. 8 denotes a variation of a gate voltage Vg. In these figures, A represents a case in which a pulse duration tw is long, and B shows a case in which a pulse duration tw is long. When the current Iak is larger than 3000 A and the pulse duration tw is longer than several tens xcexcs, breakdown of the reverse conducting static induction thyristor does not occur. However, when a pulse duration tw is set to a shorter value within a range from several hundreds ns to several xcexcs, the reverse conducting static induction thyristor might be broken. In this case, a breakdown point situates in the static induction thyristor section and no abnormal phenomenon occurs in the diode section. From these phenomena, it is assumed that the breakdown of the reverse conducting static induction thyristor depends on an inclination of a raising portion of the current Iak. In the longer pulse duration shown in FIG. 6A, an inclination of a reverse current ir (dir/dt) is about 0.5 KA/xcexcs, and in the shorter pulse duration illustrated in FIG. 6B, an inclination of the reverse current is about 3 KA/xcexcs. Furthermore, as depicted in FIG. 8B, when the breakdown of the reverse conducting static induction thyristor due to discharge failure occurs, a remarkable variation appears in the gate voltage Vg immediately after a reverse voltage peak.
Next the performance of the diode upon an occurrence of an abruptly increase in the current flowing through the diode is analyzed. FIGS. 9 and 10 show a forward current IF flowing through the anode-cathode path of the diode 32 shown in FIG. 4 and a forward voltage drop VF appearing across the anode-cathode path of the diode when the diode is operated by a pulse. A denotes a case of a smaller inclination and B represents a case of s larger inclination. From these graphs it can be understood that there is an intimate correlation between the inclination of the raising portion of the current IF and a transient on-voltage (forward recovery voltage) VFP as shown in FIG. 11. That is to say, for the diode having the breakdown voltage of 4000 V, when the inclination of the current IF (dIF/dt) is about 500 A/xcexcs, the forward recovery voltage VFP is lower such as about 70 V, but when the inclination of the current (dIF/dt) is high such as 1000 A/xcexcs and 2000 A/xcexcs, the forward recovery voltage VFP is becomes higher such as about 100 V and 170 V, respectively.
FIG. 12 is a graph showing a relationship between the forward recovery voltage VFP and the breakdown voltage of the diode for the inclination dIF/dt of 2000 A/xcexcs. In accordance with the increase in the diode breakdown voltage, the forward recovery voltage VFP becomes higher. When the diode has a breakdown voltage of 4000 V, the forward recovery voltage VFP is about 170 V. In the reverse conducting static induction thyristor, the breakdown voltage of the diode section should be not lower than the breakdown voltage of the thyristor section, and therefore the diode section should have the breakdown voltage of several thousands volts. The diode section having such a high breakdown voltage also has a high forward recovery voltage VFP. In other words, the higher the breakdown voltage of the diode section is, the forward pulse current hardly flows through the diode section.
In the manner explained above, in the known reverse conducting static induction thyristor having a breakdown voltage of, for instance 4 KV, when a large reverse current is to flow immediately after conducting a large forward current, the protection diode section could not be made conductive, and a large amount of carriers stored in the channel regions in FIG. 5 flow abruptly in the reverse direction from the cathode region 46 to the buried gate region 43. Particularly, in a region denoted by X in FIG. 5, i.e. in a vicinity of the buried gate region 43 into which the gate current is supplied much more abruptly than the central gate region 42, an excessive amount of carriers are generated and there is produced a filamentation between the channel regions by the diode reverse recovery phenomenon of the diode section and the thyristor section 44 might be destroyed.
It should be noted that the above explained problem occurs not only in the reverse conducting static induction thyristor, but also in a semiconductor switching device such as normal type thyristor, gate turn-off (GTO) SCR and insulated gate bipolar transistor (IGBT).
The present invention has for its object to provide a novel and useful semiconductor device, in which the above mentioned problem of the known reverse conducting static induction thyristor, and even if a high reverse voltage is applied to a switching element abruptly, a protection diode can be brought into conductive and the switching element can be effectively protected from the breakdown.
According to the invention, a semiconductor device having reverse conducting faculty comprises:
a switching element including a semiconductor substrate of a first conductivity type having first and second major surfaces, a first main electrode region of the first conductivity type formed in the first major surface of the semiconductor substrate, a first main electrode connected to said first main electrode region, a second main electrode region of a second conductivity type formed in the second major surface of the semiconductor substrate, a second main electrode connected to said second main electrode region, a control electrode region of the second conductivity type formed in the first major surface of the semiconductor substrate for controlling a current passing between the first and second main electrode regions, and a control electrode connected to said control region; and
a series arrangement of a plurality of diodes connected between said first main electrode and said second main electrode in an opposite polarity to a current flowing between said first main electrode region and said second main electrode region, each of said plurality of diodes having a breakdown voltage lower than a breakdown voltage of said switching element.
Upon practicing the semiconductor device according to the present invention, it is preferable that said series arrangement of a plurality of diodes is formed in said first major surface of the semiconductor substrate in which said first main electrode region is also formed. Such a structure is particularly suitable for a high frequency pulse circuit in which inductance of wiring has to be reduced as far as possible. According to the invention, said series arrangement of a plurality of diodes may be formed on a separate semiconductor substrate from said semiconductor substrate semiconductor substrate which constitutes said switching element, or said series arrangement of a plurality of diodes may be formed as a diode stack including first and second electrodes connected to said first and second main electrodes of the switching element, respectively.
In the latter two cases, it is preferable that said switching device and series arrangement of a plurality of diodes are installed in a common package in view of a reduction of wiring inductance. However, according to the invention, said switching device and series arrangement of a plurality of diodes may be in separate packages.
In a preferable embodiment of the semiconductor device according to the present invention, said switching element is formed as a static induction thyristor whose cathode region and cathode electrode are formed by said first main electrode region and first main electrode, respectively, whose anode region and anode electrode are formed by said second main electrode region and second main electrode, respectively, and whose gate region and gate electrode are formed by said control region and control electrode, respectively.
In another preferable embodiment of the switching device according to the invention, said switching element is formed as a reverse conducting static induction thyristor including
thyristor section whose cathode region and cathode electrode are formed by said first main electrode region and first main electrode, respectively, whose anode region and anode electrode are formed by said second main electrode region and second main electrode, respectively, and whose gate region and gate electrode are formed by said control region and control electrode, respectively; and
a main diode section having an anode region connected to said cathode electrode of the thyristor section and a cathode region connected to said anode electrode of the thyristor section.
In these preferable embodiments, said series arrangement of a plurality of diodes are preferably formed as field limiting rings surrounding said static induction thyristor. In this case, a plurality of diodes of said series arrangement may be preferably formed such that breakdown voltages of the diodes are gradually increased toward outside.